1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, and more particularly to a valve timing control system for an internal combustion engine which controls valve timing of the engine by changing a cam phase while learning a reference value indicative of a reference position of the cam phase.
2. Description of the Prior Art
Conventionally, a valve timing control system of this kind for an internal combustion engine (hereinafter simply referred to as xe2x80x9cthe enginexe2x80x9d) was proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 6-299876. This valve timing control system includes a cam phase change mechanism that is capable of changing the valve timing by advancing or retarding the angle of a phase of a cam shaft relative to a crankshaft (cam phase), a crank angle sensor for detecting a crank angle, a cam sensor for detecting a cam angle, and an ECU for controlling the cam phase change mechanism based on outputs from these sensors. This valve timing control system calculates a cam phase from values of the crank angle and the cam angle detected during idling when the cam phase change mechanism is not in operation, and the cam phase is most retarded, calculates a cam phase from values of the crank angle and the cam angle detected when the cam phase change mechanism is in operation, and calculates an actual cam phase as a difference between these cam phases. Then, from a difference between a desired cam phase determined based on engine operation conditions with reference to a predetermined table, and the actual cam phase, a feedback control value for controlling the cam phase change mechanism is determined, and a drive signal based on the feedback control value is supplied to the cam phase change mechanism to control the same.
As described above, according to the conventional valve timing control system, the actual cam phase is determined from a difference between a cam phase detected when the cam phase change mechanism is in a non-operating condition in which the cam phase is most retarded and a cam phase detected when the same is in operation. More specifically, the cam angle detected when the cam phase is most retarded is only defined as a reference position (zero point) of the cam phase. Therefore, when values of the crank angle and the cam angle detected by the crank angle sensor and cam angle sensor contain errors, or when rotation of the engine is unstable, the zero point is varied and hence a correct zero point cannot be obtained. This prevents proper control of the cam phase change mechanism. Of course, it is possible to obtain a correct zero point by learning the zero point while taking into errors in detection by the sensors and changes in operating condition of the engine. However, if the learned value of the zero point stored in the RAM is lost e.g. due to replacement of a battery and instantaneous shut-down of the power supply to the ECU, it takes time to learn the zero point again. This makes it impossible to control the cam phase change mechanism by using a correct learned value of the zero point, resulting in degraded fuel economy and emission characteristics.
It is an object of the invention to provide a valve timing control system for an internal combustion engine, which is capable of properly setting the reference position of the cam phase by learning, and at the same time promptly establishing a learned value of the reference position when the learned value is lost, thereby accurately changing the cam phase for proper control of the valve timing of the engine.
To attain the above object, the invention provides a valve timing control system for an internal combustion engine having a crankshaft, an intake cam, and an exhaust cam, the valve timing control system controlling valve timing of the engine by changing a cam phase of at least one of the intake cam and the exhaust cam relative to the crankshaft.
The valve timing control system according to the invention is characterized by comprising:
a cam phase change mechanism for changing the cam phase;
cam phase detection means for detecting the cam phase;
reference position-learning means for learning a reference value indicative of a reference position of the cam phase at a predetermined learning speed based on the cam phase detected by the cam phase detection means when the cam phase change mechanism is in a predetermined operative state;
storage means for storing therein the reference value learned by the reference position learning means;
reference value loss detection means for detecting whether or not the reference value is lost from the storage means; and
learning speed-correcting means for correcting a learning speed at which the reference position-learning means learns the reference value such that the learning speed is increased from the predetermined learning speed for a predetermined time period, when the reference value loss detection means detects loss of the reference value from the storage means.
According to this valve timing control system, normally, based on the cam phase detected by the cam phase detection means when the cam phase change mechanism is in a predetermined operative state, the reference position-learning means learns the reference value indicative of the reference position of the cam phase at the predetermined learning speed. This learning makes it possible to properly set the reference value indicative of the reference position of the cam phase. Further, when the reference value loss detection means detects the loss of the reference value from the storage means, the learning speed correction means corrects the learning speed at which the reference value is learned such that the learning speed is increased from the predetermined learning speed, for the predetermined time period. This makes it possible to establish the reference value (learned value) promptly when the learned reference value is lost from the storage means. As a result, it is possible to properly change the cam phase, to thereby properly control the valve timing of the internal combustion engine.
Preferably, the storage means comprises a first RAM, and the reference value loss detection means is a counter implemented by a second RAM that is provided separately from the first RAM and is battery-backed-up, the counter detecting the loss of the reference value from the first RAM due to interruption of supply of electric current to the first RAM by being reset to a predetermined value when the interruption of the supply of electric current to the first RAM occurs.
Preferably, the predetermined operative state of the cam phase change mechanism is a state in which the cam phase change mechanism is not in operation and the cam-phase is most retarded.
Preferably, the valve timing control system includes operating condition determination means for determining whether or not the engine is in a predetermined operating condition suitable for learning of the reference value, and reference value learning-permitting means for permitting the learning of the reference value when the operating condition determination means determines that the engine is in the predetermined operating condition.
More preferably, the predetermined operating condition of the engine is a condition in which a predetermined time period has elapsed after a start of the engine, the intake cam is in a most retarded state, and a rotational speed of the engine is equal to or larger than a predetermined value, and a variation in the rotational speed of the engine is lower than a predetermined level.
Preferably, the learning speed correction means comprises a downcount timer which is set to a predetermined time period when the reference value loss detection means detects the loss of the reference value from the storage means, and coefficient-setting means for setting an averaging coefficient applied to an average value-calculating equation used for the learning of the reference value to a value corresponding to a larger leaning speed than the predetermined learning speed, before the downcount timer counts down to zero.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.